Tube bundles of tubes having high heat-conductivity, such as of aluminum or copper, are well known in the art. For example, such materials are used as heat exchangers in air conditioning units, both for the evaporator section and for the condenser section. Generally, these tube bundles are formed by mounting a plurality of thin-walled tubes, parallel to each other, in two or more support plates. Actual mounting of the tube bundles is via the support plate.
In use, a heat transfer fluid passes through these tubes, generally with a plurality of passes through a single bundle. Thus, the fluid will flow in one direction in one tube, and in a reverse direction through another tube in the bundle. Obviously, under these circumstances, it is required that the tubes be joined together in pairs to allow for this reverse flow. Such joining is accomplished by attaching a reverse bend to a pair of tubes. This is far more economical than taking a single length of tubing and bending it to provide the necessary return bends. Further, bending of the tube is impractical in view of the fins which must be placed on the tube and, further, in view of the fact that normally more than one layer of tubes is employed. The joint formed between the reverse bend and the straight section of tubing must be fluidtight to prevent leakage of the heat transfer fluid which passes through the tubes. A variety of methods have existed in the prior art for accomplishing this joining, including the use of ultrasonic soldering.
Ultrasonic soldering provides a number of advantages over other soldering methods known to the art, as is apparent to those skilled in the art. However, there are problems with ultrasonic soldering, particularly if the two members being soldered to each other are not at the proper temperature. With too low a temperature, it may not be possible to form an acceptable bond. Particularly with ultrasonic soldering, however, if the temperature is too high, there is a great danger of disintegrating the thin-walled tubes in the ultrasonic soldering bath. The prior art has not dealt with these two distinct problems in an economical manner. A preferred method of dealing with the problem is to assure a relatively constant, equal temperature on all parts of the tube assembly, prior to its insertion into the ultrasonic soldering bath.
In the patent to Rosales, U.S. Pat. No. 2,417,662, a method for brazing similar tube bundles is set forth, though that brazing method does not involve an ultrasonic process. Rather, Rosales deals with preheating of the tube bundles, dipping the preheated bundles into a flux to clean the adjacent parts, and then brazing the materials. Rosales assures a constant temperature on the portions of the tube bundle to be brazed in the preheat step. This is not an economical solution.